The present invention relates to a drop-on-demand ink-jet printing head for jetting ink, in the form of small droplets, from an ink reservoir so as to form printed dots on recording paper.
Drop-on-demand ink-jet printing head can be classified into three main types. The first type is a so-called bubble jet type in which a heater for instantaneously vaporizing ink is provided on the top end of a nozzle to thereby produce and jet an ink drop by expansion pressure created during vaporization. In the second type, a piezoelectric element provided in a vessel constituting an ink reservoir flexes or expands in accordance with an electrical signal applied thereto so as to jet ink in the form of a drop by a force produced when the element expands. In the third type, a piezoelectric element is provided in an ink reservoir in opposition to a nozzle so as to jet an ink drop by dynamic pressure produced in a nozzle area upon expansion of the piezoelectric element.
As disclosed in Japanese Patent Publication No. Sho-60-8953, the above-mentioned third type drop-on-demand ink-jet printing head has a configuration wherein a plurality of nozzle apertures are formed in a wall of a vessel constituting an ink tank, and piezoelectric elements are disposed at the respective nozzle apertures matched in the direction of their expansion and contraction with each other.
In this printing head, a printing signal is applied to the piezoelectric elements so as to selectively actuate the piezoelectric elements to jet ink drops from the corresponding nozzles by the dynamic force produced when the piezoelectric elements are actuated to thereby form dots on printing paper.
In such a printing head, it is desirable that the efficiency in ink drop formation and the force of ink drop jetting are large. However, since the unit length of a piezoelectric element and the rate of expansion/contraction of the same per unit voltage are extremely small, it is necessary to apply a high voltage in order to obtain sufficient jetting force for printing, and it is therefore necessary to construct a driving circuit and electric insulators so as to withstand such a high voltage.
In order to obtain a high jetting force, European Patent Unexamined Publication No. 372521 discloses a drop-on-demand ink-jet printing head in which a piezoelectric plate is fixedly attached to an elastic metal plate and is cut and divided corresponding to the arrangement of nozzle apertures, with one end of the piezoelectric plate being fixed to a frame while the other end thereof opposite to the nozzle apertures is a free end.
In this printing head, a driving signal is applied to the piezoelectric plate to thereby bend the elastic metal plate to store energy. In this state, the application of the driving signal is stopped to thereby release the elastic force stored in the elastic metal plate so that dynamic pressure is applied to ink, creating a repulsion force to thereby discharge the ink in the form of ink drops to the outside through the nozzle apertures.
However, there is a problem in that a high voltage has to be applied to the piezoelectric plate to bend the elastic metal plate to such an extent as to form ink drops.